This invention relates generally to a class of oversampling digital-to-analog converters, known as delta-sigma converters. More particularly, this invention relates to an oversampling digital-to-analog converter with high linearity and low quantization error.
Oversampling digital-to-analog (D/A) converters have become increasingly popular because they avoid many of the difficulties encountered with conventional D/A converters. Conventional converters have attributes that make their circuits difficult to implement in fine-line VLSI technology. Chief among these is the use of analog filters, the need for high-precision analog circuits, and the vulnerability of conventional D/A converters to noise and interference. On the other hand, the virtue of conventional D/A converters is their use of a relatively low sampling frequency, usually the Nyquist rate of the signal (i.e., twice the rate of the signal bandwidth).
oversampling delta-sigma D/A converters typically contain a digital interpolation filter (IF) stage followed by a noise-shaping loop (NL), an internal single-bit D/A converter and an analog low-pass filter (LPF) for extracting the original signal. The IF stage samples the digital input signal at an increased sampling rate. The NL stage quantizes the oversampling data into a single-bit data stream. The single-bit data stream is converted into an analog signal by the single-bit D/A converter, which is inherently linear and thus does not distort the input signal in the conversion process. The analog output signal of the single-bit D/A converter contains the desired analog signal. However, the output signal of the D/A converter also contains a large amount of out-of-band quantization noise which must be removed by the analog low-pass filter.
Because of the nature of the out-of-band quantization noise, the realization of the analog low-pass filter is complex and difficult. The quantized signal is a density-modulated square wave with a large amplitude as well as steep slopes. Moreover, the linearity of the low-pass filter is also difficult to maintain in the presence of such a large and fast-changing signal.
One way to reduce the complexity of the low-pass filter is to utilize a multibit rather than a single-bit data stream and multibit internal D/A converter. Every extra bit in the data stream reduces the quantization noise by about six decibels (dB) and also reduces the rate of change of the signal. However, the multibit D/A converter is directly in the signal path, and hence any nonlinearity in its characteristics introduces harmonic distortion and additional noise into the passband of the input signal.
The present invention achieves the linear output provided by a single-bit D/A converter while reducing the quantization noise and rate of change of the signal to that obtainable by a multibit D/A converter. The design requirements for the low-pass filter are thus considerably reduced and its linearity more easily maintained.